The present invention relates to a seal strip for a hollow circular cylindrical screening device, which screening device is intended to be used in a paper or cardboard machine for the production of paper or cardboard, said seal strip exhibiting indicator means for indicating wear of the seal strip. The seal strip is e.g. intended to be used to seal between a vacuum box which is arranged inside the screening device, and the mantle of the screening device.
In the production of paper or cardboard, a continuous process for the dewatering of a paper pulp of cellulosic fibres is used, in order to form a sheet material. In connection with the dewatering there is thereby used so called suction rolls, i.e. hollow, rotating, circular cylindrical screening devices, in a paper or cardboard machine. Such suction rolls may be of different types, e.g. formation rolls which control the formation of the sheet, pick-up rolls which guide the pulp web, couch rolls or press suction rolls. One example where a suction roll is used is a suction roll where the paper pulp is spread over the length of the suction roll and forms a web around at least a part of the circumference of the suction roll, whereby liquid is sucked from the paper pulp and through the perforated mantle of the suction roll. A vacuum box is thereby stationary arranged inside the suction roll, along a circular arc of its circumference and along its entire length, whereby an underpressure in the vacuum box leads to liquid being forcedly sucked out from the paper pulp when it passes over the area of the vacuum box. Usually there is also arranged a press roll in order to press out liquid from the paper pulp, in a press nip with contact between the suction roll and the press roll. Dewatering in a press nip is typically performed before the paper pulp web is brought to pass over the area of the vacuum box.
The vacuum box, which is used in all types of suction rolls, is arranged to seal against the inside of the mantle surface of the suction roll by aid of two or more seal strips, which are arranged in parallel to each other, with an angular distance which corresponds to the extension of the vacuum box. When more than two seal strips are used, the vacuum box may be divided into several vacuum zones. The seal strips exhibit an arc shaped sealing surface, at least after having been used a certain time period, and slide against the mantle surface at the inside of the screening device as it rotates, whereby the seal strip is gradually worn during the continuous operation. At the short ends of the vacuum box, there is arranged gable sealings. In order to limit the wear and in order to give an enhanced sealing, water is normally sprayed against the mantle surface on the inside of the screening device in a position before the first seal strip as seen in the direction of rotation. The seal strips are nevertheless worn and must accordingly be replaced at even intervals. Since the paper machine is operating continuously, it may take several months before it is stopped for service, including checking of the seal strips which are being used. Quite often, they are thereby replaced either at closer intervals than necessary or far too seldom. The first case is uneconomical, since the seal strips are thrown away before their life time is finished and the second case is uneconomical since the efficiency of the suction roll decreases at operation with too worn seal strips. During operation, there is no possibility to check the wear.
In U.S. Pat. No. 5,746,891, there is presented wear indicators for seal strips for suction rolls, which wear indicators are checked when the plant is closed down and which at that point give an indication as to whether the wear allowance of the seal indicators has been used up or if there is additional life time left. According to one embodiment, the wear indicators consist of bores in the sealing surface, which bores are spaced apart and filled with a coloured material, whereby the material may exhibit different colours at different depths. The colour which appears at the time of the check thereby indicates how much wear allowance is left. According to another embodiment, isolated electrical conductors are arranged in intervals along the seal strip, inside the same. When the seal strip has been worn all the way down to these conductors, the insulation on the conductors are worn off too, and there is formed an electrical circuit, via the liquid in the system, whereby a signal lamp is illuminated.
The drawback of the seal strips shown in U.S. Pat. No. 5,746,891 is that there, in the first embodiment, is no control of the wear at all until the plant is stopped for service, whereby at service, at least the machine clothing must be taken off or whereby in practice, the roll has to be opened. In the second embodiment, there is at the other hand only an indication when the signal lamp is illuminated. If this lamp is illuminated e.g. only a short time period after the plant having been stopped for service, this means that the plant must be stopped again for change of the seal strip. Accordingly, the seal strips according to U.S. Pat. No. 5,746,891 do not give an opportunity of continuous surveillance of the remaining wear allowance of the strips.
The object of the present invention is to solve the above mentioned problems, whereby there is presented a seal strip with indicator means arranged to continuously give a measure of a remaining wear allowance if the seal strip.
According to one aspect of the invention, said indicator means or at least a transmitter therefore, is mounted inside the seal strip, which seal strip at least in the main consists of a magnetically non-conducting material, preferably graphite, whereby the indicator means is arranged to continuously measure a distance between the indicator means, or the transmitter, and a surface against which the seal strip is arranged to seal.
According to another aspect of the invention, said indicator means include measuring based on electromagnetism. The methods of measurement may thereby be the per se known methods reluctance measurement, inductive measurement or eddy current measurement, reluctance measurement being the most preferred. Reluctance measurement is advantageous since it is a method by which the measurements are adequately precise, stable and independent of material and temperature variations in the mantle of the screening device. If a lower preciseness and stability can be accepted, the measurement may however be performed by inductive or eddy current method.
According to another aspect of the invention, there may instead be used a level indicator as indicator means, preferably of the potentiometer type, which is arranged to measure the position of a fixed point at or in the seal strip. Thereby, the level indicator continuously shows how the seal strip is displaced upwards as the wearing takes place (the displacement is explained in greater detail in the description of the figure), whereby the displacement and thereby also the value measured by the level indicator may indirectly and continuously give a measure of the remaining allowance.
According to yet another aspect of the invention, there is offered a possibility to continuously measure and thereby to continuously give a measure of the wear rate of the seal strip in mm/hour or rather xcexcm/hour. The system may also be provided with a warning signal, e.g. in the form of a lamp or tone, whereby the operator""s attention may be drawn to an increased wear rate so that he can undertake the necessary actions in order to decrease the wear rate, e.g. by increased liquid spray etc. In order to be able to get a measure of the wear rate, there is required a precise measurement, in the form of reluctance measurements.
According to another aspect of the invention, a temperature transmitter is arranged in connection with the indicator means or its transmitter, inside the seal strip. This temperature transmitter is continuously giving a measure of the temperature, whereby a warning signal may be given e.g. when the temperature exceeds 100xc2x0 C., which is only the case if there is a liquid shortage. Then, the operator may increase the liquid spray or take other necessary precautions in order to lower the temperature.
The great advantage with the seal strip according to the invention is that a continuous measure of the remaining wear allowance may be given, which measure is precise and stable. Other advantages are the possibility to get a continuous measure of wear rate and temperature.
Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.